Activation effect on carbon nanotubes

ABSTRACT

Particles, which may include nanoparticles, are mixed with carbon nanotubes and deposited on a substrate to form a cold cathode. The particles enhance the field emission characteristics of the carbon nanotubes. An additional activation step may be performed on the deposited carbon nanotube mixture to further enhance the emission of electrons.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to the following U.S.Provisional Patent Applications, Ser. Nos. 60/343,642, filed Oct. 19,2001, 60/348,856, filed Jan. 15, 2002, and 60/369,794 filed Apr. 4,2002.

TECHNICAL FIELD

The present invention relates in general to carbon nanotubes, and inparticular, to the utilization of carbon nanotubes in field emissionapplications.

BACKGROUND INFORMATION

Carbon nanotubes have been used by many for field emission applications.Carbon nanotubes (CNTs) come in two families, single wall nanotubes(SWNTs) and multi-wall nanotubes (MWNTs). Both materials are long(11–10,000 microns) and thin (0.001–0.1 microns in diameter). This highaspect ratio and the fact that they are semiconducting or metallic makesthem ideal candidates for field emission applications. One problem,however, is if the CNTs are too densely packed, the CNTs shield eachother from the strong electrical fields needed to extract the electronsfrom the material. The field emission from these materials is furtherimproved if the CNT fibers are aligned in parallel to the appliedelectrical field. Also desired is an inexpensive way of applying the CNTmaterial onto suitable substrate materials at low temperature andaligning these materials using methods that are suitable for large-scalemanufacturing.

By growing CNTs directly on a catalyst, some success has been achievedin growing the CNT materials with acceptable density and alignment, butnot always in a predictable fashion. Furthermore, the growthtemperatures are high, too high for using low-temperature sodalime glassthat is commonly used in the display industry.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a process in accordance with an embodiment of thepresent invention;

FIG. 2 illustrates a process for applying carbon nanotubes to asubstrate;

FIGS. 3A–3G illustrate a process in accordance with an embodiment of thepresent invention;

FIG. 4 illustrates a field emission image of a sample made with aprocess in accordance with the present invention;

FIG. 5 illustrates a field emission image of a sample made with one ofthe embodiments in accordance with the present invention;

FIGS. 6A–6E illustrate a process in accordance with the presentinvention for applying carbon nanotubes to a substrate;

FIG. 7 illustrates a data processing system configured in accordancewith the present invention;

FIG. 8 illustrates another process for activating an electron sourcematerial in accordance with an embodiment of the present invention;

FIG. 9 illustrates carbon nanotubes on a silicon wafer applied in apaste;

FIG. 10 illustrates carbon nanotubes on a silicon wafer applied in apaste and activated;

FIG. 11 illustrates carbon nanotubes applied using a spray method;

FIG. 12 illustrates a graph of current versus electrical field foractivated and non-activated carbon nanotubes;

FIG. 13 illustrates an image of emission sites of non-activated pixels;and

FIG. 14 illustrates an image of emission sites from activated pixels inaccordance with the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forthsuch as specific cathode configurations to provide a thoroughunderstanding of the present invention. However, it will be obvious tothose skilled in the art that the present invention may be practicedwithout such specific details. In other instances, well-known circuitshave been shown in block diagram form in order not to obscure thepresent invention in unnecessary detail. For the most part, detailsconcerning timing considerations and the like have been omitted in asmuch as such details are not necessary to obtain a completeunderstanding of the present invention and are within the skills ofpersons of ordinary skill in the relevant art.

Refer now to the drawings wherein depicted elements are not necessarilyshown to scale and wherein like or similar elements are designated bythe same reference numeral through the several views.

The present invention provides a method of applying CNT materials ontoalmost any substrate material and activating for field emission the CNTfibers in a reproducible and inexpensive manner.

The source of carbon nanotube powders can be purified single-wall carbonnanotube (SWNT) powders from Carbon Nanotechnologies, Inc. (Part # HPR92S13). These SWNTs were 1 nm in diameter and 100˜1000 nm in length. But,any other kinds of single wall or multiwall carbon nanotubes can also beused in this method. There is no need to purify the CNT materials toeliminate the catalyst from the carbon.

One method is to grind the CNT materials into shorter lengths. Thisallows better control of material properties. In some cases,satisfactory results may be achieved without grinding. A typical ballmill was used to grind CNT bundles. FIG. 1 is the schematic diagram ofsuch a ball mill. The rate of this machine is about 50˜60 revolutionsper minute. In this method, 0.2 g CNT bundles as well as 40–100 Al₂ 0 ₃balls (5˜10 mm in diameter) were mixed into 200˜300 ml IPA (Isopropylalcohol). The mixture was ground for 1˜7 days in order to disperse theCNTs. A surfactant (Triton® X-100, about 1 drop per 100 ml IPA) or otherkind of materials can also be added to the mixture in order to achievebetter dispersion of CNTs.

Other solvents can be used instead of IPA (e.g., acetone). Mixtures ofsolvents can also be used. Water or mixtures of water and solvent mayalso be used. IPA is inexpensive, is not extremely hazardous or toxic,and can be dried at relatively low temperatures.

Because the CNTs can easily agglomerate (stick to each other), anultrasonic mixing process was applied to the CNT solution to dispersethe CNTs again before spraying them onto the substrates. An ultrasonatormade by (Sonics and Materials Inc., Danbury, Conn.) was used to furtherdisperse the carbon nanotubes. Full power for 3–5 minutes, until the IPAstarts to warm to about 40C. Other means of applying ultrasonic energyto the solution may also be tried.

Next, the process involves a spraying of the CNT mixture onto thesubstrate. In this method, the CNT mixture can be sprayed on variouskinds of substrates such as metal, ceramic, glass, plastics, organic andsemiconductors. The substrates can be coated with conducting, insulatingor semiconducting patterned layers to provide electrical conductivity tosome areas and electrical isolation or selected electrical resistance toother areas. These layers can be deposited using printing methods (thickfilm) or by evaporation, sputtering or other thin film methods. Standardphotolithography patterning and/or etching processes may be needed foradditional patterning of the added layers.

Referring to FIG. 2, in order to get more uniform and well dispersed CNTsolution coating on the substrates, more IPA can be added into the abovesolution before spraying. In this method, the CNT solution 201 for spraycan be approximately 0.05 g CNT in 1000 ml IPA. Condensed gas 203 cancharge an atomizer 202 to create the spray. CNT mixture 206 can besprayed on selected areas by using a shadow mask 205. In order toprevent the solution 206 from flowing to unexpected areas, the substrate204 can be heated up to 50C–100C both on the front side and back sideduring the spray process. The substrate 204 can be sprayed back andforth or up and down several times until the CNT mixture 206 covers theentire surface uniformly. The thickness of the CNTs 206 may be about 1˜2μm. Then they are dried in air naturally or using a heat lamp 207.

Ink jet printing or other printing techniques (or any other depositionprocess) may also be used to apply the CNT mixture to the substrate. Inkjet processes have advantages in a large scale manufacturingenvironment.

After the CNTs are sprayed on the substrate, a taping process may beused to remove some of the CNTs from the surface. In this method, 3MScotch tape may be used to remove CNTs from the surface. But, many othervarieties of tape can be used in this process. The tape is adhered onthe CNT coating. It is important to be sure that there is no air betweenthe tape and the CNT coating. If air exists between them, the CNTs atthat area will not be removed. A rubber roller can be used to furtherpress the tape in order to eliminate air gaps in the interface. Finally,the tape is removed by pulling up at one end. A very thin CNT layer isleft on the substrate.

FIGS. 3A–3G illustrate in further detail the foregoing process. In FIG.3A, a substrate 301 is cleaned. In FIG. 3B, conductive (e.g., feedlines302 are added to substrate 301 by using printing methods. In FIG. 3C, ashadow mask 303 is added, wherein the holes in the shadow mask 303 arealigned to the areas of the substrate where it is desired to deposit theCNT material. A magnet 304 may be used to hold the shadow mask 303 tothe substrate 301. In FIG. 3D, the foregoing spraying process (see FIG.2) is used to spray on the CNT mixture 305. The solvent in the mixture305 evaporates leaving the CNT material 305. A heater 306 is applied tothe back of the substrate 301 and magnet 304, and alternatively, a heatlamp (not shown) may also be used on the front, to speed the evaporationprocess and to keep the mixture from running under the mask 303.

FIG. 3E shows the cold cathode after the mask 303, magnet 304 and heater306 are removed. The CNT material 305 is patterned on the feedlines 302.The CNT material 305 may be left to dry further if required.

FIG. 3F shows the application of tape 309 to the surface of the cathodewith the adhesive of the tape in contact with the CNT material 305. Thetape 309 may be applied to a tape substrate 308. Rolling of the tape maybe used to further press the tape 309 onto the CNT material 305 using acompliant roller 307.

FIG. 3G shows the removing of the tape 309. This can be done by pullingup from one end to the other of the substrate backing 308. Portions 310of the CNT material 305 are thus removed with the tape 309 leaving theCNT materials 305 on the feedlines 302 aligned. The tape 309 can bediscarded.

A field emission image of a sample cold cathode created by this processis shown in FIG. 4.

The technique of mixing carbon nanotubes with host materials such asadhesives of all kind is known (sometime this is called “carbonnanotubes in a paste”). This paste is generally printed (for example,screen printed) on a substrate to define localized emission spots. Inthese emission spots, carbon nanotubes are homogeneously mixed with thepaste. In the virgin situation after printing, the carbon nanotubespossess a random orientation on the paste, meaning that a large part ofthe nanotubes are oriented at different angles with respect to thevertical of the substrate, but also many other carbon nanotubes aredistributed similarly around a line parallel to the substrate. As aresult, the contribution to field emission of these carbon nanotubesthat are not oriented vertically with respect to the substrate isminimal or null. Furthermore, the existence of a high concentration ofcarbon fibers in the material and the random orientation can createnon-optimized electric field distribution in the paste including thecarbon nanotubes and as a result shielding effects between neighboringnanotubes.

It is desirable to have a process whereby one can re-align thesenanotubes, mechanically or otherwise, and also would be very importantto lower in some cases the density of the carbon nanotubes in order tolower the shielding effect in an active device. This process can beimplemented utilizing existing soft adhesives in the sense that asustaining substrate that is coated with these soft adhesives can beapplied to the surface of the printed paste including carbon nanotubessuch that in a pulling process using the above soft adhesives, one canexercise suitable force on the carbon nanotubes to achieve the followingresults:

-   -   a) Increase the concentration of carbon nanotubes aligned        vertically or with the small distribution with respect to the        normal to the substrate;    -   b) Pull some of the carbon nanotubes totally out of the mixture        to achieve optimal carbon nanotube surface density; and    -   c) By using an optimal soft adhesive minimizing the surface        contamination of the emissive area still achieving results a and        b above.

Excellent emission results can be obtained utilizing this technology.FIG. 5 illustrates a field emission image of a cold cathode samplecreated with the “carbon nanotubes in a paste” process described infurther detail with respect to FIGS. 6A–6B. In FIG. 6A, a substrate 601is cleaned. In FIG. 6B, conductive (e.g., metal) feedlines 602 aredeposited on substrate 601 using printing methods. FIG. 6C shows themixture of CNT material mixed with a paste 603 printed in a pattern onthe feedlines 602. The paste 603 may consist of CNT material, silverpaste, glass frit, a glass frit vehicle, and a glass frit thinner. Anexample of this paste is 0.5 grams CNT material, 1.4 grams frit vehicle,and 1.25 grams of silver paste (silver paste may be a Dupont product7713, Conductor Composition; frit vehicle may be a Daejoo VehicleDJB-715 from Daejoo Fine Chemical Co., Ltd., or from Pierce and StevensF1016A02; CNT material may be provided by Carbon Nanotechnologies, Inc.,purified or unpurified (CNT material may be multi-wall or single-wall)).

In FIG. 6D, an adhesive tape 604 applied to a backing 605 may be appliedto the surface of the cathode such that the adhesive 604 of the tape isin contact with the CNT paste material 603. A roller 606 may be used toapply uniform contact pressure. FIG. 6E shows the tape 604 being removedby peeling from one side to the other. Some of the CNT material paste607 is pulled off with the tape 604. In this process, the CNT fibers arealigned in the vertical direction, and the density of the CNT fibers 603not aligned is reduced.

An alternative process is the utilization of single wall or multi-wallor a mixture of single wall and multi-wall carbon nanotubes in IPA(alcohol or other solvent) host. Furthermore, in order to homogenize thesolution of carbon nanotubes and IPA, certain chemicals are added to themixture in order to diminish the surface forces between the carbonnanotubes and obtain isolated carbon nanotubes in a homogeneous mixturewith the IPA with minimal bundles (aggregates or clusters of carbonnanotubes all together).

An advantage of this method is that by obtaining this homogeneousmixture, a spraying process can be utilized through a mechanical orother kind of mask such that spraying this mixture directly onto theactive substrate through the mask will localize the carbon nanotubes onthe future emission sites, and the fixation of these carbon nanotubeswill be achieved by spraying onto the substrate where the substratetemperature is 50–100 degrees C.

Furthermore, after the fixation of the carbon nanotubes on the desiredemissive locations, the same pulling technique can be used, but thistime the pulling forces will be exercised more uniformly on all thecarbon nanotubes that are exposed in the first layers on the sprayedmaterial. As a result the aligning process and the decrease in thedensity of the carbon nanotubes is more efficient, more effective andmore controllable.

Referring to FIG. 8, there is illustrated an alternative embodimentwhereby a patterned, or embossed, activation surface is used to activatethe electron source material in a manner as described previously. Oftenthe cathode plate with the electron source is not a flat surface, whichlends itself to activation from a flat activation surface (e.g.,adhesive tape). Therefore, when the tape is applied, it may not be ableto adequately activate the electron source material, which in thisexample is the carbon nanotubes. To address the problem, an embossedactivation surface can be used so that the adhesive is able to reachdown to the electron source material at each pixel site. Moreover, areasthat do not need to be activated are not subject to contact with theadhesive material. Note, such a patterned activation surface can be usedwithout embossing where only certain areas need to be activated. In FIG.8, a substrate 801 has a conductive cathode 802 pattern thereon, andpatterned insulators 804 with metal gates 805 deposited thereon. Carbonnanotubes 803 are then deposited onto the cathodes 802. To activate thecarbon nanotubes 803, the adhesive 807 may be embossed onto a backingfilm 806 so that the adhesive 807 reaches down to the carbon nanotubes803 within the insulator 804 walls.

A representative hardware environment for practicing the presentinvention is depicted in FIG. 7, which illustrates an exemplary hardwareconfiguration of data processing system 713 in accordance the subjectinvention having central processing unit (CPU) 710, such as aconventional microprocessor, and a number of other units interconnectedvia system bus 712. Data processing system 713 includes random accessmemory (RAM) 714, read only memory (ROM) 716, and input/output (I/O)adapter 718 for connecting peripheral devices such as disk units 720 andtape drives 740 to bus 712, user interface adapter 722 for connectingkeyboard 724, Mouse 726, and/or other user interface devices such as atouch screen device (not shown) to bus 712, communication adapter 734for connecting data processing system 713 to a data processing network,and display adapter 736 for connecting bus 712 to display device 738.CPU 710 may include other circuitry not shown, which will includecircuitry commonly found within a microprocessor, e.g., execution unit,bus interface unit, arithmetic logic unit, etc. CPU 710 may also resideon a single integrated circuit. Display device 738 can implement thedisplay technology described herein.

Either utilizing CVD to grow nanotubes or spraying or mixing nanotubesinto a paste, then applying to a substrate for electron field emissions,it appears that electron emission current is strongly related to carbonnanotube density when applied onto substrates. It has been found that byactivating the surface (e.g., by using adhesive tape to remove somecarbon nanotube material) better electron emission characteristics canbe achieved. For example, four samples of spray and paste carbonnanotubes on silicon wafers were made, one of the wafers activated fromeach group and one wafer kept as control. These were then inspected andnanotubes counted per square area from high power SEM pictures. FIG. 9illustrates the SEM picture from the paste control wafer, while FIG. 10illustrates the paste-activated wafer. FIG. 11 illustrates thespray-activated wafer. The following table shows the CNT density persquare centimeter for each of the samples.

CNT DENSITY PER SQUARE CENTIMETER Paste- Paste- Control ActivatedSpray-Control Spray-Activated Wafer Wafer Wafer Wafer ~1 × 10¹⁰ ~1 × 10⁹~1 × 10¹⁰ − 1 × 10¹¹ ~1 × 10⁸ − 1 × 10⁹

Referring to FIG. 12, the emission from the activated devices was muchbetter than that of the non-activated devices at a given electric field.The SEM pictures have shown that the carbon nanotubes density afteractivated is around 1%–10% of the non-activated samples. The fieldemission is improved if the CNT density per square centimeter is lessthan 1×10¹⁰. Thus, the emission current is inversely proportional to thecarbon nanotube density. FIG. 13 shows field emission from sites ofnon-activated pixels, while FIG. 14 shows light emission from sites ofactivated pixels.

1. A method for making a field emission cathode comprising the steps of: depositing a carbon nanotube (“CNT”) mixture on a cathode structure using a spraying process; contacting the deposited CNT mixture with an adhesive material; and separating the adhesive material from the deposited CNT mixture.
 2. The method as recited in claim 1, wherein the separating step further comprises the step of removing a portion of the deposited CNT mixture from the cathode structure.
 3. The method as recited in claim 1, wherein the CNT mixture deposited in the depositing step has a CNT concentration of 1×10¹⁰ CNT density per square centimeter or greater, and wherein the CNT mixture on the cathode structure after the separating step has a CNT concentration of less than 1×10¹⁰ CNT density per square centimeter.
 4. The method as recited in claim 1, wherein the CNT mixture on the cathode structure after the separating step has more alignment of CNTs substantially at a normal to a surface of cathode structure than the CNT mixture depositing in the depositing step before the separating step.
 5. A method for making a field emission cathode comprising the steps of: depositing a carbon nanotube (“CNT”) mixture on a cathode structure using a spraying process; and removing a portion of the deposited CNT mixture from the cathode structure.
 6. The method as recited in claim 5, wherein the CNT mixture deposited in the depositing step has a CNT concentration of 1×10¹⁰ CNT density per square centimeter or greater, and wherein the CNT mixture on the cathode structure after the removing step has a CNT concentration of less than 1×10¹⁰ CNT density per square centimeter.
 7. The method as recited in claim 5, wherein the CNT mixture on the cathode structure after the removing step has more alignment of CNTs substantially at a normal to a surface of cathode structure than the CNT mixture deposited in the depositing step before the removing step.
 8. A method for making a field emission cathode comprising the steps of: depositing a carbon nanotube (“CNT”) mixture on a cathode structure; contacting the deposited CNT mixture with an adhesive material; and separating the adhesive material from the deposited CNT mixture, wherein the CNT mixture is deposited in a plurality of wells in the cathode structure and wherein the adhesive material is attached to a backing film in a pattern that matches a pattern of the plurality of wells in the cathode structure so that when the contacting step is performed, the backing film is laid over the cathode structure so that the adhesive material inserts into the plurality of wells so that the adhesive material contacts the CNT mixture. 